The present invention relates to a network element and method for combining ATM (ATM=Asynchronous Transfer Mode) and TDM (TDM=Time Division Multiplexed) traffic on a link, preferably a PDH line (PDH=Plesiochronous Digital Hierarchy), like an E1, T1 or JT1-line, of a communication network.
New mobile networks using ATM transmission, so called third Generation or 3G networks, are built up besides existing GSM networks. GSM networks using TDM transmission are called second generation or 2G networks herein. The 3G network elements will not totally replace the 2G network elements. Both technologies complement one another and may share network infrastructure like transmission lines.
Network elements can be interconnected via star and/or chain topology. The present invention relates to a network comprising a mixed sequence of 2G- and 3G-network elements interconnected via chain topology. That requires a coexistence of ATM-traffic and TDM-traffic on the same line, because the 3G network elements use ATM traffic whereas 2G network elements handle TDM-traffic.
A combined 2G-3G mobile network installed in chain topology and including for instance a sequence of 3G-2G-3G network elements, has coexisting ATM-traffic and TDM-traffic on that line which connects the first 3G-network element performing the port of chain to the networks mean parts and the following 2G network element in the chain. A transmission of ATM and TDM traffic simultaneously over the same physical line (e.g. E1) requires an interface that handles ATM traffic and TDM traffic at the same time.
An unpublished possibility shown in FIG. 1 is to use an additional time slot multiplexer 7 for generating a new frame that contains TDM traffic in some time slots and the content of ATM cells in other time slots of a common frame.
Basically, PDH-interfaces of a 3G-element can be configured either for transmission of ATM cells over an ATM N×64 kbit/s PDH circuit according the recommendation AF-PHY-0130.00 or for tunnelling a N×64 kbit/s circuit through an ATM network by using Circuit Emulation Service (CES) according recommendation AF-VTOA-0078.000.
The ATM Forum recommendation: AF-VTOA-0078.000 “Circuit Emulation Service (CES) Interoperability Specification V 2.0”, January 1997, part of the Voice and Multimedia over ATM Specifications, describes the support of Constant Bit Rate (CBR) traffic over ATM-networks. A Structured N×64 and Unstructured DS1/E1/J2 services described herein offer different ways to connect DS1/E1/J2 equipment across emulated circuits carried on an ATM network. The Structured DS1/E1/J2 N×64 service is modelled after a Fractional DS1/E1/J2 circuit. According to this Document an ATM Circuit Emulation Service (CES) Interworking Function (IWF) connects an ATM network via physical interfaces to standard constant bit-rate (CBR) circuits (e.g., DS1/DS3, J2 or E1/E3). The job of a CES Interworking Function is to extend the constant bit-rate circuit to which it is connected across the ATM network. This is done in a manner that is transparent to the terminating equipment of the CBR circuit.
In contrast, the ATM Forum recommendation AF-PHY-0130.00:    “ATM on Fractional T1/E1”, part of the Physical Layer Specifications, October 1999, specifies the requirements to map a ATM cell stream on a “circuit-mode connection” made up of an integer number of 64 kbit/s channels up to the maximum number supported on a DS1 or E1 interface.    The transmission convergence sub-layer (TC) is carried on an N×64 kbit/s channel complying with I.231.10 (i.e., it supports octet alignment of an unrestricted octet stream with sequence integrity).